Actuator, light quantity adjusting apparatus, and stepping motor

ABSTRACT

To provide an actuator which maintains high efficiency, can react to an increase in pole, has a high part fit, and can be manufactured at low cost while it is thinned. The actuator includes: a magnet which is formed in a ring shape, in which at least one surface perpendicular to a central axis thereof is divided in a peripheral direction and alternately plane-magnetized in different poles; a stator including a plurality of magnetic pole teeth opposed to the magnetized surface of the magnet; a rotor which is held to be rotatable about a rotational axis as a center, includes a disk portion whose surface perpendicular to the rotational axis is bonded to the magnet, and made of a soft magnetic material; and a coil which is located on substantially the same surface as that of the magnet, fixed to the stator, and excites the stator and the disk portion.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an actuator which is suitably mountedas a drive source for a small electronic device such as a camera, alight quantity adjusting apparatus using the actuator, and a steppingmotor to which principle of the actuator is applied.

2. Related Background Art

FIG. 18 shows a conventional shutter apparatus for a lens shuttercamera.

In FIG. 18, reference 101 denotes a permanent magnet, 102 denotes adriving lever, and 102 a denotes a driving pin provided in the drivinglever 102. The driving lever 102 is fixed to the permanent magnet 101and integrally rotates with the permanent magnet 101. Reference numeral103 denotes a coil and 104 and 105 denote stators which are made of asoft magnetic material and excited through the coil 103. The stator 104and the stator 105 are connected with each other through portions 104 aand 105 a, so that the stator 104 and the stator 105 are integrallyformed in view of a magnetic circuit. When a current is supplied to thecoil 103, the stator 104 and the stator 105 are excited, with the resultthat the permanent magnet 101 rotates within a predetermined anglerange. Reference numerals 106 and 107 denote shutter blades and 108denotes a base plate having an opening portion 108 a. Hole portions 106a and 107 a of the shutter blades 106 and 107 are rotatably fit to pins108 b and 108 c of the base plate 108. The driving pin 102 a is slidablyinserted into long holes 106 b and 107 b of the shutter blades 106 and107. When the driving lever 102 rotates with the permanent magnet 101,the shutter blades 106 and 107 pivot about the hole portions 106 a and107 a, so that an aperture which is not shown is opened or closed.

As another mode, there is also a shutter apparatus having a structure inwhich a permanent magnet is made of plastic and integrally formed with adriving pin in order to prevent an increase in cost.

Reference numeral 109 denotes a front base plate for holding the shutterblades 106 and 107 so that the blades are movable between the front baseplate 109 and the base plate 108. Reference numeral 110 denotes a rearbase plate for holding the stators 104 and 105 and holding the permanentmagnet 101 so that the magnet is rotatable.

The use of a digital camera has been widely spread. The digital cameraperforms photoelectric conversion on an image of a field to be imagedusing a CCD or the like as an image pickup device and causes a recordingmedium to record a converted image as still image information. Theoperation of the digital camera of this type with respect to exposurewill be briefly described below.

First, a main power source is turned on before photographing. When theimage pickup device becomes an operating state, the shutter blades areheld at an open position in which the image pickup device can beexposed. Therefore, the accumulation of charge and the transfer(emission) thereof are repeated in the image pickup device, with theresult that the field to be imaged can be observed through an imagemonitor.

After that, when a release bottom is pressed, a diaphragm value and anexposure time are determined according to an output from the imagepickup device at this time. Then, whether or not it is necessary tonarrow the diameter of an exposure aperture is determined based ondetermined results. When it is necessary to narrow the diameter of theexposure aperture, the shutter blades are driven to set a predetermineddiaphragm value. Next, an instruction for starting the accumulation ofcharge (accumulation start signal) is provided to the image pickupdevice from which the accumulated charge has emitted. Simultaneously, anexposure time control circuit is activated in response to theaccumulation start signal serving as a trigger signal. After a lapse ofa predetermined exposure time, the shutter blades are driven to a closedposition in which exposure to the image pickup device is blocked. Afterthe exposure to the image pickup device is blocked, the accumulatedcharge is transferred and image information is recorded on a recordingmedium through an image writing device. The blocking of exposure to theimage pickup device during the transfer of charge is intended forpreventing a charge quantity from changing by unnecessary light duringthe transfer of charge.

In addition to the above-mentioned shutter apparatus, there are ashutter apparatus having a mechanism for inserting and removing an NDfilter into and from an optical path and a shutter apparatus having amechanism for inserting and removing a diaphragm regulation memberhaving a small diaphragm diameter into and from an optical path.

In the conventional shutter apparatus, a coil and stators take up agreat deal of base plate space. Therefore, it is hard to locate anotheractuator, a guide rod for a lens, and the like. In view of this point,the following light quantity adjusting apparatus is proposed in JapanesePatent Application Laid-open No. 2002-049076 of the application filed bythe present applicant.

FIG. 19 is an exploded perspective view showing a light quantityadjusting apparatus disclosed in Japanese Patent Application Laid-openNo. 2002-049076. FIG. 20 is a sectional view showing the light quantityadjusting apparatus shown in FIG. 19 in an axial direction. The lightquantity adjusting apparatus includes an actuator serving as a drivingdevice. The actuator has a magnet 201, a coil 202, a stator 203, anauxiliary stator 204, and a blade driving pin 201 h. The magnet 201 isrotatable around the center of rotation as an axis and at least theouter peripheral surface thereof is divided in a peripheral direction soas to be alternately magnetized in different poles. The coil 202 islocated in the axial direction of the magnet 201. In the stator 203, anoutside magnetic pole portion 203 a and an inside magnetic pole portion203 b which are excited by the coil 202 are opposed to the outer andinner peripheral surfaces of the magnet 201. The auxiliary stator 204 isfixed to the inside magnetic pole portion 203 b of the stator 203 andexcited by the coil 202. The blade driving pin 201 h is integrallyformed with the magnet 201. The light quantity adjusting apparatusfurther includes a base plate 205 with an opening portion 205 a andlight quantity control blades 207 and 208. The light quantity controlblades 207 and 208 are driven by the blade driving pin 201 h of theactuator so as to adjust an opening quantity of the opening portion 205a of the base plate 205. Reference numeral 206 denotes a retainingplate.

When the light quantity adjusting apparatus having the above-mentionedstructure is used, the coil and the magnet are located in the axialdirection. Therefore, there is a large effect that the light quantityadjusting apparatus becomes a compact apparatus in which the coil andthe magnet do not take up a great deal of base plate space.

When an additional reduction in size of a digital camera or the likeusing the above-mentioned actuator is demanded, it is necessary toachieve slimming of the digital camera in addition to a reduction insize of the actuator itself. However, in the above-mentioned actuator,the coil and the magnet are located in the axial direction. Therefore,extreme slimming is difficult because efficiency of the actuator islikely to reduce owing to a reduction in magnetization efficiency of themagnet in the peripheral direction.

On the other hand, the fundamentals of an actuator having a rotatingshaft are also applied to a stepping motor.

FIG. 21 is a partially sectional view showing a two-phase stepping motordisclosed in Japanese Patent Publication No. H06-083561 as an example ofa conventional technique. The stepping motor includes a set of permanentmagnets 301 and 302 overlapped with each other through a disk magneticmaterial 303 fixed to a rotating shaft 311. The permanent magnets 301and 302 are formed such that a plurality of magnetic poles formed bymagnetization in the axial direction alternately become differentmagnetic poles in the peripheral direction. An inner tooth 305 radiallyprotruding outward and an outer tooth 306 radially protruding inward areformed in each of the stators 307 and 308 which are opposed to thepermanent magnets 301 and 302, respectively. The stators 307 and 308 areprovided with excitation coils 309 and 310 for exciting the stators 307and 308, respectively and positioned on both sides of the permanentmagnets 301 and 302 such that the stators 307 and 308 are shifted fromeach other by an electrical angle of (½)π.

Here, a magnetic flux passes through a housing 317 surrounding theexcitation coil 309, the outer tooth 306, the permanent magnet 301, themagnetic material 303, the permanent magnet 302, and the inner tooth 305in order and then returns to the housing 317. The permanent magnet 301is a magnet. When the inner tooth 305 and the outer tooth 306 whichserve as an inlet and outlet of the magnetic flux are intended to opposeto the plane-magnetized magnets, the inner tooth 305 and the outer tooth306 are actually on substantially the same plane. The magnetic material303 serves as merely a back metal and is not excited by the excitationcoils 309 and 310. Constituent elements required for the motor are (a)the housing 317 (also serving as a yoke), (b) the excitation coil 309,(c) the inner tooth 305 and the outer tooth 306 (magnet pole teeth), (d)the magnet (permanent magnet 301), (e) the magnetic material 303, (f)the magnet (permanent magnet 302), (g) the inner tooth 305 and the outertooth 306 (magnet pole teeth), (h) the excitation coil 310, and (i) thehousing 318 (also serving as a yoke), which are shown in order along theaxial direction.

As described above, according to the stepping motor described inJapanese Patent Publication No. H06-083561, a large number of elementsare located so as to overlap with the magnets having the surfacesperpendicular to the shaft in the axial direction. Therefore, a lengthof the stepping motor in the axial direction lengthens, so that it ishard to thin the stepping motor. In addition, the inner tooth and theouter tooth are provided in each of the stators and located such thatthe inner tooth and the outer tooth are shifted from each other by anelectrical angle n in the case where a distance between the samemagnetic poles in the peripheral direction of the magnets is given asone electrical angle. Therefore, when the number of magnetic poles ofthe stepping motor is intended to increase, a width of each of magneticpole teeth and a gap between adjacent magnetic pole teeth becomenarrower, so that the difficulty in part processing increases and aphysical strength reduces. Thus, the development of a stepping motorwhich can be thinned and is capable of dealing with an increase in polehas been desired.

FIGS. 22 and 23 show a two-phase stepping motor disclosed in JapanesePatent Publication No. H06-083564 as another example of the conventionaltechnique. FIG. 22 is an exploded perspective view showing the two-phasestepping motor and FIG. 23 is a sectional view after assembly. Thestepping motor includes a rotating shaft 403, a first permanent magnet401, and a second permanent magnet 402. The first permanent magnet 401and the second permanent magnet 402 are fixed to the rotating shaft 403and magnetized in a thickness direction. In each of the first permanentmagnet 401 and the second permanent magnet 402, different magnetic polesare alternately formed in a peripheral direction. A first stator 406includes: a set of magnetic pole teeth 404 and 405 which are opposed toboth sides of the magnetic poles of the first permanent magnet 401; andan excitation coil 410. A second stator 409 includes: a set of magneticpole teeth 407 and 408 which are opposed to both surfaces of themagnetic poles of the second permanent magnet 402; and an excitationcoil 411. The magnetic pole teeth 404 and 405 of the first stator 406and the magnetic pole teeth 407 and 408 of the second stator 409 arepositioned such that the first stator 406 and the second stator 409 areshifted from each other in the peripheral direction by an electricalangle of (½)π. When they are assumed to be constituent elements of amagnetic circuit, the magnetic circuit becomes a six-layer structurewhich includes the magnetic pole tooth 404 of the first stator 406, thefirst permanent magnet 401, the magnetic pole tooth 405, the magneticpole tooth 408, the second permanent magnet 402, and the magnetic poletooth 407 of the second stator 409. The number of air gaps between eachof the stators and each of the magnets is two per phase, that is, fourin total. Note that the stator is a stator.

As described above, the stepping motor described in Japanese PatentPublication No. H06-083564 is composed of a large number of partsprovided in a multi-layer form. Therefore, a length of the steppingmotor in the axial direction lengthens, so that it is hard to thin thestepping motor and a cost of parts increases. A large number of partsare provided in the multi-layer form and overlapped with one another inthe axial direction. In addition, the number of air gaps between thestators and magnets, by which a magnetic characteristic is significantlyinfluenced, is four, so that the parts cannot be easily assembled withhigh precision. It is necessary to pass the central rotating shaftthrough a large number of parts in order during assembly, so that anassembly time lengthens and it is likely to increase an assembly cost.The number of parts is large, so that it is likely to reduce assemblyprecision due to the accumulation of part processing precision. Thus, itis hard to construct a high performance stepping motor.

A stepping motor having a short shaft is disclosed as anotherconventional technique (Japanese Patent Application Laid-open No.H02-058035). FIG. 24 shows a structural example of the stepping motorprovided as a driving source for an exposure quantity adjustingapparatus, which is disclosed in Japanese Patent Application Laid-openNo. H02-058035. A surface of a magnet 505 which is perpendicular to anaxial direction is divided in a peripheral direction and magnetized. Twocoils 506 and 507 are located on both sides of the magnet 505. Themagnet 505 and the coils 506 and 507 are sandwiched by two stators 508and 509 in a shift direction. Therefore, it is possible to suitablyincorporate such a thin stepping motor into the exposure quantityadjusting apparatus for a camera.

However, there are the following problems in the structural example.First, the pole teeth per phase of the stators 508 and 509 which areopposed to the magnet 505 correspond to about half of the entirecircumference, so that it is possible to use only a magnetic flux equalto or smaller than half of an effective magnetic flux of the magnet.Therefore, it cannot be said that the above-mentioned structure iseffective. Second, the coils are close to the stators in only upper andlower ends, so that the amount of leakage magnetic flux from the coilsto the circumference is large. Therefore, it cannot be said that theabove-mentioned structure is effective. Third, the pole teeth of thestators which are opposed to the magnet correspond to about half of theentire circumference as described above, so that a range in which adriving force is caused in each excited phase also corresponds to abouthalf of the entire circumference. Therefore, when the driving force isconverted into torque, an unnecessary force in a transverse direction islikely to cause, with the result that there are concerns with respect tovibration, noise, non-uniform rotation, and a reduction in positionalprecision in the stepping motor.

As described above, the conventional actuators and stepping motors havethe following problems. The number of parts is large, thereby increasinga cost. It is hard to achieve slimming. High efficiency is hardlyobtained. It is hard to deal with an increase in number of poles. Partscannot be easily fit.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide an actuator whichmaintains high efficiency, can react to an increase in pole, has a highpart fit, and can be manufactured at low cost while it is thinned, and alight quantity adjusting apparatus using the actuator as a drivingsource.

A second object of the present invention is to provide a highperformance stepping motor which can react to an increase in pole, has ahigh part fit, and can be manufactured at low cost while it is thinned.

To achieve the first object, according to first to fourth aspects of thepresent invention, there is provided an actuator, including: a magnetwhich is formed in a ring shape, in which at least one surfaceperpendicular to a central axis thereof is divided in a peripheraldirection and alternately plane-magnetized in different poles; a statorincluding a plurality of magnetic pole teeth opposed to the magnetizedsurface of the magnet; a rotor which is held to be rotatable about arotational axis as a center, includes a disk portion whose surfaceperpendicular to the rotational axis is bonded to the magnet, and madeof a soft magnetic material; and a coil which is located onsubstantially the same surface as that of the magnet, fixed to thestator, and excites the stator and the disk portion.

According to the actuator having the above-mentioned structure, the coiland the magnet are located on substantially the same surface. Forexample, the stator to which the coil is fixed and the rotor to whichthe magnet is fixed each are formed in a thin ring shape, so that theentire apparatus can be thinned. Although the actuator in which themagnet is opposed to the stator in the axial direction to form a closedmagnetic path is used, the magnet is fixed to the disk portion of therotor, with the result that the number of types of air gap between thestator and the magnet becomes only one. Therefore, precision betweenparts forming the air gap is high, so that assembly is easy. The numberof necessary parts into which a shaft is inserted is small, so thatassembly is easier. Magnetic fluxes generated in the disk portion of therotor and the stator by energization to the coil effectively act on themagnet located between the disk portion of the rotor and the stator. Arotating output of the rotor to which the magnet is fixed becomeshigher.

In the first to fourth aspects of the present invention, the magnet ispreferably fixed to a surface of the disk portion of the rotor which isopposed to the stator on an outer peripheral side and the coil ispreferably fixed to a surface of the stator which is opposed to themagnet on an inner peripheral side.

According to the actuator having the above-mentioned structure, themagnetic flux generated by the coil passes through the stator adjacentto the coil, the rotor, and the disk portion. Therefore, a magneticresistance is small and a leakage magnetic flax reduces. The magnet islocated on the outer peripheral side of the actuator and the coil islocated on the inner peripheral side thereof. Therefore, a diameter ofthe magnet becomes larger, with the result that output torque increases.

In the first to fourth aspects of the present invention, the actuatorpreferably further includes a bearing which is fixed to the stator,rotatably supports a shaft portion of the rotor, and made of a softmagnetic material.

According to the actuator having the above-mentioned structure, thebearing is made of the soft magnetic material having a small magneticresistance. Therefore, a bearing portion can be also included in aneffective magnetic circuit, so that magnetic saturation of the magneticcircuit can be prevented.

In the first to fourth aspects of the present invention, the rotor ispreferably positioned in an axial direction by making a tip protrudingportion of the shaft portion to be in contact with a bottom side of thebearing and the bearing is preferably extended such that a tip endsurface thereof is close to the disk portion of the rotor.

According to the actuator having the above-mentioned structure, therotor is in contact with the tip protruding portion of the shaft portionfor rotational sliding, having small friction and a small contactdiameter. Therefore, even when attraction in the axial direction islarge, the influence of the friction can be reduced. The stator fixed tothe bearing is constantly attracted to the rotor in the axial directionthrough the magnet, with the result that the rotor is resistant todisconnect in an opposite direction to the bearing. Therefore, it ispossible to perform positioning in the axial direction and a radialdirection using the single bearing. The tip end surface of the bearingis made close to the disk portion of the rotor, so that it can be usedas a magnetic path having a small magnetic resistance. Thus, magneticsaturation is unlikely to cause.

In the first to fourth aspects of the present invention, the actuatorpreferably further includes a dummy yoke which is located outside themagnet, bonded to a cover made of a non-magnetic material, and made of asoft magnetic material.

According to the actuator having the above-mentioned structure, when theactuator has the cover to which the dummy yoke is bonded, torque whichacts on the magnet in a case other than the case where the stator isexcited, so-called cogging torque can be adjusted according to acharacteristic of the actuator. For example, retaining torque of therotor at each of start and end positions of rotation of the rotor can beincreased without a change in a rotational characteristic of the rotorat the time of energization to the coil.

Furthermore, to achieve the first object, according to the first tofourth aspects of the present invention, there is provided a lightquantity adjusting apparatus, including: a magnet which is formed in aring shape, in which at least one surface perpendicular to a centralaxis thereof is divided in a peripheral direction and alternatelyplane-magnetized in different poles; a stator including a plurality ofmagnetic pole teeth opposed to the magnetized surface of the magnet; arotor which is held to be rotatable about a rotational axis as a center,includes a disk portion whose surface perpendicular to the rotationalaxis is bonded to the magnet, and made of a soft magnetic material; acoil which is located on substantially the same surface as that of themagnet, fixed to the stator, and excites the stator and the diskportion; an output member which operates in response to rotation of therotor; a base plate including an opening portion; and a light quantityadjusting member which is driven by the output member and moves abovethe opening portion of the base plate to change a quantity of lightpassing through the opening portion.

According to the light quantity adjusting apparatus having theabove-mentioned structure, a quantity of light passing through theopening portion can be changed by using the actuator as a drivingsource. Therefore, the light quantity adjusting apparatus itself can bethinned.

Furthermore, to achieve the second object, according to fifth to eighthaspects of the present invention, there is provided a stepping motor,including: a first magnet and a second magnet, each of which is formedin a ring shape, in each of which at least one surface perpendicular toa central axis thereof is divided in a peripheral direction andalternately plane-magnetized in different poles; a first statorincluding a plurality of magnetic pole teeth which are opposed to themagnetized surface of the first magnet and extended in a radialdirection; a second stator including a plurality of magnetic pole teethwhich are opposed to the magnetized surface of the second magnet andextended in the radial direction; a rotor which is made of a softmagnetic material and includes a shaft portion held to be rotatable anda disk portion having a surface perpendicular to the shaft portion; afirst coil which is fixed to the first stator and excites the firststator and the disk portion of the rotor to which the first magnet isfixed on one side; and a second coil which is fixed to the second statorand excites the second stator and the disk portion of the rotor to whichthe second magnet is fixed on the other side.

According to the stepping motor having the above-mentioned structure, athree-layer structure (upper, middle, and lower layers) has the firststator to which the first coil is fixed, the second stator to which thesecond coil is fixed, and the rotor to which the first magnet and thesecond magnet are fixed and which is interposed between the first statorand the second stator, so that the stepping motor can be thinned.Although a two-phase stepping motor is used, the number of types of airgap between the stator and the magnet is only two. Therefore, highprecision assembly is easy. The number of necessary parts into which ashaft is inserted is small, so that assembly is easier. Magnetic fluxesgenerated by energization to the first coil effectively act on the firstmagnet located between the first stator and the disk portion of therotor to which the first magnet is fixed in the axial direction.Magnetic fluxes generated by energization to the second coil effectivelyact on the second magnet located between the second stator and the diskportion of the rotor to which the second magnet is fixed in the axialdirection. Thus, a rotating output of the rotor to which the firstmagnet and the second magnet are fixed becomes higher.

In the fifth to eighth aspects of the present invention, it ispreferable that a first flange portion which protrudes from the diskportion to the first stator be located inside the first coil fixed tothe first stator, the first magnet be located outside the first coil,and the disk portion of the rotor be located on an opposite side of thefirst coil which is in contact with the first stator, a second flangeportion which protrudes from the disk portion to the second stator belocated inside the second coil fixed to the second stator, the secondmagnet be located outside the second coil, and the disk portion of therotor be located on an opposite side of the second coil which is incontact with the second stator.

According to the stepping motor having the above-mentioned structure,the magnetic fluxes generated by the first coil and the second coil passthrough the first and second stators which are adjacent to those coilsand each made of the soft magnetic material, the first and second flangeportions, and the disk portion of the rotor. Therefore, a magneticresistance is small and a leakage magnetic flax reduces. The firstmagnet and the second magnet are located on an outer peripheral side ofthe stepping motor, and the first coil and the second coil are locatedon an inner peripheral side thereof. Therefore, a diameter of each ofthe magnets becomes larger, with the result that output torqueincreases.

In the fifth to eighth aspects of the present invention, the steppingmotor preferably further includes a bearing which is fixed to the firststator and the second stator, rotatably supports the rotor, and made ofa soft magnetic material, the bearing being connected with the firstflange portion and the second flange portion.

According to the stepping motor having the above-mentioned structure,the bearing is made of the soft magnetic material having a smallmagnetic resistance. Therefore, a bearing portion can be also includedin an effective magnetic circuit, so that magnetic saturation of themagnetic circuit can be prevented.

The above and other objects, features, and advantages of the inventionwill become more apparent from the following detailed description takenin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view showing an actuator according toa first embodiment of the present invention;

FIG. 2 is an exploded perspective view showing a light quantityadjusting apparatus using the actuator according to the first embodimentof the present invention;

FIG. 3 is a sectional view in an axial direction, showing the lightquantity adjusting apparatus shown in FIG. 2;

FIGS. 4A and 4B are bottom views showing the actuator in respectivestates in which a magnet is rotated counterclockwise and clockwise byswitching energization to a coil;

FIG. 5 is an exploded perspective view showing an actuator according toa second embodiment of the present invention;

FIG. 6 is a sectional view in an axial direction, showing an actuatoraccording to a third embodiment of the present invention;

FIG. 7 is a sectional view in an axial direction, showing an actuatoraccording to a fourth embodiment of the present invention;

FIG. 8 is a bottom view showing the actuator according to the fourthembodiment of the present invention;

FIG. 9 is an exploded perspective view showing a stepping motoraccording to a fifth embodiment of the present invention;

FIG. 10 is a sectional view in an axial direction, showing the steppingmotor shown in FIG. 9;

FIG. 11 is a schematic side view showing a phase relationship between amagnet and a stator in the stepping motor shown in FIG. 9;

FIG. 12 is a schematic side view showing a state in which the magnet isrotated by 18 degrees from a state shown in FIG. 11 by switchingenergization to a coil;

FIG. 13 is a schematic side view showing a state in which the magnet isrotated by 18 degrees from the state shown in FIG. 12 by switchingenergization to the coil;

FIG. 14 is a schematic side view showing a state in which the magnet isrotated by 18 degrees from the state shown in FIG. 13 by switchingenergization to the coil;

FIG. 15 is a sectional view in an axial direction, showing a steppingmotor according to a sixth embodiment of the present invention;

FIG. 16 is a sectional view in an axial direction, showing a steppingmotor according to a seventh embodiment of the present invention;

FIG. 17 is a sectional view in an axial direction, showing a steppingmotor according to an eighth embodiment of the present invention;

FIG. 18 is an exploded perspective view showing an example of aconventional shutter apparatus;

FIG. 19 is an exploded perspective view showing another example of aconventional shutter apparatus;

FIG. 20 is a sectional view showing a light quantity adjusting apparatusshown in FIG. 19;

FIG. 21 is a sectional view in an axial direction, showing aconventional stepping motor;

FIG. 22 is an exploded perspective view showing another conventionalstepping motor;

FIG. 23 is a sectional view in an axial direction, showing the steppingmotor shown in FIG. 22; and

FIG. 24 is a perspective view showing an exposure quantity adjustingapparatus provided with another conventional thin stepping motor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described in detail withreference to the accompanying drawings.

Actuators and light quantity adjusting apparatuses according to thepresent invention are described below in a first embodiment to a fourthembodiment. Stepping motors according to the present invention aredescribed below in a fifth embodiment to an eighth embodiment.

First Embodiment

FIGS. 1 to 4 show an actuator and a light quantity adjusting apparatusaccording to a first embodiment of the present invention. Morespecifically, FIG. 1 is an exploded perspective view showing theactuator. FIG. 2 is an exploded perspective view showing a lightquantity adjusting apparatus in which the actuator shown in FIG. 1 isincorporated as a driving source. FIG. 3 is a sectional view in an axialdirection, showing the light quantity adjusting apparatus. FIGS. 4A and4B are bottom views of B-B in FIG. 3, showing a state of rotationoperation of a magnet. FIG. 4A shows a magnet and a stator in a state inwhich the magnet is rotated counterclockwise when viewed from a baseplate side. FIG. 4B shows the magnet and the stator in a state in whichthe magnet is rotated clockwise by 30 degrees from the state shown inFIG. 4A by switching energization to the coil.

In FIGS. 1 to 4A and 4B, a ring magnet 1 has magnetized portions whichare formed by dividing a surface of the magnet 1 which is perpendicularto the central axis (virtual axis) thereof into P (P is an even number;P=8 in this embodiment) in a peripheral direction. The magnetizedportions are alternately plane-magnetized in the N-pole and the S-pole.Here, the surface of the magnet 1 which is opposed to a stator 3described later, that is, an opposite rear surface to a surface of themagnet 1 which can be viewed in FIG. 1 is magnetized. As shown in FIGS.4A and 4B, the magnetized portions 1 a, 1 c, 1 e, and 1 g are magnetizedin the S-pole and the magnetized portions 1 b, 1 d, 1 f, and 1 h aremagnetized in the N-pole. The magnet 1 is formed by compressing a bondedneodymium magnet. The magnet 1 may be formed by injection molding ormade of a ferrite system material. A coil 2 is formed by winding a leadwire about a bobbin made of an insulating material and fixed to thestator 3 described later at a substantially concentric position to themagnet 1 on an inner peripheral surface side of the magnet 1. The coil 2is connected with an outside through terminal portions 2 a.

The stator 3 is made of a soft magnetic material and produced by aforging press process or the like. The stator 3 includes: magnetic poleteeth 3 a, 3 b, 3 c, and 3 d which protrude in a comb-tooth shape from asurface perpendicular to the central axis thereof in an axial direction;and a hole 3 e to which a bearing 4 described later is fixed. The numberof magnetic pole teeth is ½ of the number of poles (P) of the magnetizedmagnet 1, that is, 4. All the magnetic pole teeth 3 a to 3 d of thestator 3 are excited in the same pole (N-pole or S-pole) by energizationto the coil 2.

The bearing 4 is fixed into the hole 3 e of the stator 3 to smoothlyrotate a rotor 5 described later. The bearing 4 is made of metal whichis a soft magnetic material such as electromagnetic soft iron by, forexample, a sintering manufacturing method. This material is desirably anoilless type. A bottom portion 4 a is a thrust abutment having a highsliding property. The bottom portion 4 a is in contact with ahemispherical portion 5 d of the rotor 5 described later to position themagnet 1 relative to the stator 3 in an axial direction, so that thebottom portion 4 a becomes the center of rotation. A flat portion 4 b isopposed to a disk portion 5 b of the rotor 5 described later and isclose to the disk portion 5 b so as not to be in contact with the diskportion 5 b.

The rotor 5 is composed of a shaft portion 5 a rotatably supported bythe bearing 4, the disk portion 5 b having a surface orthogonal to theshaft portion 5 a, an output pin 5 c, and the hemispherical portion 5 dformed at the tip of the shaft portion 5 a. The rotor 5 is made of asoft magnetic material such as electromagnetic soft iron, having a smallmagnetic resistance. An opposite side surface to the magnetized surfaceof the magnet 1 is fixed to the disk portion 5 b of the rotor 5. Theoutput pin 5 c is used to transfer the rotation of the rotor 5 to bladesdescribed later and protrudes from a cutaway portion 6 d of a base plate6 described later. The surface of the hemispherical portion 5 d issmoothly processed so as to minimize friction caused between the rotor 5fixed to the magnet 1 and the bottom portion 4 a of the bearing 4 fixedto the stator 3 by axial directional attraction caused between themagnet 1 and the magnetic pole teeth of the stator 3.

The base plate 6 for an actuator has an opening portion 6 a. Blades 7and 8 described later are pivoted above a flat surface of the base plate6. Support shafts 6 b and 6 c are used as the centers of pivot of theblades 7 and 8 described later. The cutaway portion 6 d limits arotational range of the output pin 5 c of the rotor 5 to determine anopen position and closed position of the blades 7 and 8 by the ends ofthe cutaway portion 6 d. The base plate 6 is a molded part made of, forexample, engineering plastics.

The blades 7 and 8 are light quantity adjusting members which areoperatively linked with the output pin 5 c of the rotor 5 and pivoted(inserted into and removed from an optical path) above the openingportion 6 a of the base plate 6 to adjust a quantity of light passingthrough the opening portion 6 a. The support shafts 6 b and 6 c of thebase plate 6 are inserted into holes 7 a and 8 a of the blades 7 and 8as the centers of pivot. The output pin 5 c is inserted into long holes7 b and 8 b to perform open and close operations on the opening portion6 a. A retaining plate 9 has an opening portion 9 a.

Next, an actuator in the above-mentioned structure will be described. Asshown in the sectional view of FIG. 3 (hatching indicating the sectionalview is omitted), two upper and lower units are provided. The upper unitis the stator 3 to which the bearing 4 and the coil 2 are fixed. Thelower unit is the rotor 5 whose disk portion 5 b is bonded to the magnet1. When an air gap between the magnetized surface of the magnet 1 andeach of the magnetic pole teeth of the stator 3 is given by a gap A, thegap A may be set to a minimal distance at which the magnet 1 is not incontact with the respective magnetic pole teeth even when the rotor 5rotates. On the other hand, the magnet 1 is opposed to the respectivemagnetic pole teeth of the stator 3 in the axial direction, so that theentire rotor 5 is constantly attracted to the stator 3 side. Therefore,the hemispherical portion 5 d of the rotor 5 is constantly in contactwith the bottom portion 4 a of the bearing 4 fixed to the stator 3 in asmall rotational friction state, with the result that the gap A isconstantly maintained to a predetermined distance with high precision.

In FIG. 4A, reference L denotes an angular pitch between adjacentmagnetic pole teeth of the stator 3 and M denotes a central anglecorresponding to a width of each of the magnetized portions of themagnet 1. Reference L is expressed by (360 degrees×2/P). Here, L is 90degrees. Reference M is expressed by (360 degrees/P). Here, M is 45degrees. In FIG. 4A, the rotor 5 rotates counterclockwise. The outputpin 5 c is in contact with one inner end of the cutaway portion 6 d ofthe base plate 6 and then stops. In contrast to FIG. 4A, in FIG. 4B, therotor 5 rotates clockwise. The output pin 5 c is contact with the otheropposite inner end of the cutaway portion 6 d of the base plate 6 andthen stops.

A magnetic circuit of the actuator according to the first embodiment ofthe present invention will be described with reference to FIG. 3.

When the coil 2 located in the inner circumference is energized,magnetic fluxes are generated around the coil 2. The magnetic fluxesreach the comb-tooth-shaped magnetic pole teeth 3 a, 3 b, 3 c, and 3 dof the stator 3 which are located near the coil 2 and opposed to themagnetized surface of the magnet 1. The magnetic fluxes flow from themagnetic pole teeth 3 a, 3 b, 3 c, and 3 d to the disk portion 5 b ofthe rotor 5 through the magnet 1. After that, the magnetic fluxes flowinto the center of the disk portion 5 b of the rotor 5 and transmitthrough a sectional area-into which the flat portion 4 b of the bearing4 which is opposed to the disk portion 5 b and a cross section of theshaft portion 5 a of the rotor 5 are combined. Then, the magnetic fluxesflow from the bearing 4 to the stator 3. As a result, the circulation ofthe magnetic fluxes is completed. Note that, when a sectional areaperpendicular to the direction in which the magnetic fluxes flow issmall, magnetic saturation is likely to cause in the sectional area.When only the shaft portion 5 a of the rotor 5 is used, the sectionalarea is small. Therefore, in order to prevent the magnetic saturation,the magnetic fluxes are allowed to transmit through the sectional areainto which the flat portion 4 b of the bearing 4 and the cross sectionof the shaft portion 5 a of the rotor 5 are combined. In other words, acombination of a shaft diameter of the rotor 5 and a diameter of thebearing 4 is set as a diameter of a magnetic path. The magnetic fluxesare allowed to directly flow from the disk portion 5 b close to adiameter part of the flat portion 4 b which is the tip end surface ofthe bearing 4 to the diameter part. Therefore, the diameter part is usedfor the magnetic path having a low magnetic resistance to suppress theoccurrence of the magnetic saturation. The stator 3, the disk portion 5b of the rotor 5, and the bearing 4 each are made of a soft magneticmaterial such as SUY (electromagnetic soft iron), having a smallmagnetic resistance, so that a magnetic loss is small.

Here, the reason why the actuator according to the first embodiment hasa structure in which space can be saved and high torque can be obtainedwill be described. In general, when the torque of the actuator isintended to increase without a change in supplied energy amount, it isnecessary to make a choice of a magnet having a high magnetic force andto locate the magnet in a space into which a magnetic flux flows, thatis, in a magnetic field that an magnetic path is open. On the otherhand, an opened part of a magnetic circuit has a larger magneticresistance as it widens (lengthens). Therefore, it is advantageous tominimize the part. The magnet 1 in the first embodiment of the presentinvention is plane-magnetized (alternately magnetized on the surfaceside perpendicular to the shaft in the peripheral direction). Therefore,a thickness of a magnet having a threshold thickness or more is notproportional to a magnetic force. That is, a magnet has a sufficientmagnetic force even when it is thin, so that the magnet can be thinnedto the extent a sufficient physical strength is satisfied in, forexample, manufacturing or assembling. Thus, in the first embodiment ofthe present invention, the gap A between the magnet 1 and the stator 3may be set to a minimal distance required for the rotor and the statorbased on assembly and processing precision. The magnet 1 is in directlycontact with the disk portion 5 b, so that there is no gap.

According to the actuator in the first embodiment of the presentinvention, a space which is other than the plate thickness of the stator3, the thickness of the disk portion 5 b of the rotor 5, and a minimalgap required for rotation can be used for the coil 2. As describedabove, it is unnecessary to significantly increase the thickness of themagnet 1 in many cases, so that the magnet 1 having a necessarythickness is used. The magnetic pole teeth 3 a, 3 b, 3 c, and 3 d of thestator 3 are protruded to the magnet 1 side. An air gap between each ofthe magnetic pole teeth 3 a, 3 b, 3 c, and 3 d and the disk portion 5 bof the rotor 5 is sufficiently narrowed. As a result, a magneticresistance is small. Even when a current flowing into the coil 2reduces, a large number of magnetic fluxes can be generated. Inaddition, it is possible to obtain a thin actuator having high outputand high efficiency because strong repulsion and attraction from themagnet 1 can be used. Therefore, according to the actuator in the firstembodiment of the present invention, a loss of the magnetic circuitwhich is composed of the coil 2, the stator 3, the disk portion 5 b, andthe bearing 4 for the stator 3 is small. There is no dead space. Maximalturns of wire for the coil 2 can be wound, that is, the number of turnscan be increased. Thus, torque per space is large, so that a motorhaving high efficiency is obtained.

Next, a relationship between the magnet 1 subjected to rotationaloperation and the stator 3 will be described with reference to FIGS. 4Aand 4B. The magnetized surface of the magnet 1 is opposed to themagnetic pole teeth 3 a, 3 b, 3 c, and 3 d on an opposite side of apaper surface. Therefore, “1a (rear surface S-pole)” and the like areshown in FIGS. 4A and 4B. For the sake of easy understanding, the coil 2and the like are not shown.

When the coil 2 is not energized, the magnet 1 first stops at acounterclockwise rotating position as shown in FIG. 4A. In this state, aposition of the output pin 5 c is the counterclockwise rotatingposition. The blade 7 pivots counterclockwise about the support shaft 6c as the center of pivot and the blade 8 pivots clockwise about thesupport shaft 6 b as the center of pivot. That is, the opening portion 6b of the base plate 6 is blocked, so that a shutter becomes a closedstate. For example, when the coil 2 is positively energized in theclosed state to excite the magnetic pole teeth 3 a, 3 b, 3 c, and 3 d ofthe stator 3 in the S-pole, the magnet 1 receives an electromagneticforce in the rotational direction and starts to smoothly rotateclockwise. When the magnet 1 rotates by a predetermined angle, therotation is stopped by the cutaway portion 6 d of the base plate 6 andthe energization to the coil 2 is shut off at a predetermined timing.This state is shown in FIG. 4B. The blade 7 pivots clockwise about thesupport shaft 6 c as the center of pivot and the blade 8 pivotscounterclockwise about the support shaft 6 b as the center of pivot.Then, the blades 7 and 8 are removed from the opening portion 6 a of thebase plate 6, so that the shutter becomes a full open state.

Next, when the coil 2 is reversely energized in the state shown in FIG.4B to excite the magnetic pole teeth 3 a, 3 b, 3 c, and 3 d of thestator 3 in the N-pole, the magnet 1 receives an electromagnetic forcein the rotational direction by the excitation of the magnetic pole teeth3 a, 3 b, 3 c, and 3 d and starts to smoothly rotate counterclockwise.When the magnet 1 rotates by a predetermined angle, the rotation isstopped by the cutaway portion 6 d of the base plate 6 and theenergization to the coil 2 is shut off at a predetermined timing. Thisstate is shown in FIG. 4A. The blade 7 pivots counterclockwise about thesupport shaft 6 c as the center of pivot and the blade 8 pivotsclockwise about the support shaft 6 b as the center of pivot. Then, theshutter becomes a closed state in which the opening portion 6 a of thebase plate 6 is completely blocked.

Therefore, when the energization directions to the coil 2 are switched,the light quantity adjusting apparatus serves as a shutter mechanism forblocking and unblocking light passing through the opening portion.

As described above, according to the structure of the first embodimentof the present invention, an actuator which is thin in the axialdirection without a reduction in efficiency is obtained. Magneticconstituent elements are (a) the stator 3 (magnetic pole teeth 3 a, 3 b,3 c, and 3 d), (b) the magnet 1 and the inner peripheral coil 2, and (c)the disk portion 5 b of the rotor 5, which are shown in order along theaxial direction. Thus, the actuator can be thinned.

The axial directional attraction constantly acts, so that the rotor 5 isresistant to disconnect in an opposite direction to the bearing 4.Therefore, it is possible to obtain a structure in which positioning inthe axial direction and a radial direction is performed by using thesingle bearing 4 and the disconnection to the opposite side in the axialdirection is easily prevented.

Here, the reasons why such an actuator which fundamentally includes themagnet, the coil, and the stator has a structure in which high torquecan be obtained and parts can be produced at low cost and thinnedbecause the number of parts is small will be described.

Firstly, the ring magnet 1 is fixed to the surface of the disk portion 5b of the rotor 5 made of the soft magnetic material. The stator 3excited by the coil 2 is opposed to the magnetized surface of the magnet1. As a result, the air gap between the stator 3 and the magnet 1 is setto only a gap.

Secondly, the surface of the magnet 1 which is perpendicular to thevirtual central axis thereof is divided to be alternately magnetized(plane-magnetized) in different poles.

Thirdly, a shape in which a gap between the rotor 3 and the disk portion5 b of the rotor 5 is minimized while the coil 2 is enclosed in thecomb-tooth shape in which the magnetic pole teeth 3 a, 3 b, 3 c, and 3 dof the stator 3 are extended in the axial direction. Such a shape isprovided in the entire circumference.

Fourthly, the magnet 1 is located on the external side (outer peripheralside) and the coil 2 is located inside the inner peripheral surface onsubstantially the same axis at substantially the same height.

Fifthly, the number of magnetic pole teeth 3 a, 3 b, 3 c, and 3 dprovided in the stator 3 is ½ of the number of magnetized magnetic poleson the magnet 1.

Sixthly, the single bearing 4 for the rotor 5 is provided on the stator3 side which causes attraction.

According to the above-mentioned structure, the magnetic flux generatedby the energization to the coil 2 crosses the magnet 1 located betweenthe stator 3 and the disk portion 5 b of the rotor 5, so that anelectromagnetic force is effectively caused. The magnet 1 is integrallyformed with the disk portion 5 b of the rotor 5. The coil 2 is fixed tothe stator 3. As a result, the air gap between the stator 3 and themagnet 1 is set to only a gap. Therefore, the number of parts is small.The entire actuator is composed of the three portions, so that it iseasily assembled and an assembly cost can be reduced. For example, abonded magnet used as a material of the magnet 1 is generally likely tobreak. When the bonded magnet is bonded to the rotor 5 made ofelectromagnetic soft iron or the like, a mechanical strength can beincreased. The disk portion 5 b of the rotor 5 also serves as a backmetal, so that the actuator can be maintained to a magnetically stablestate, a magnetic strength thereof is increased, and a reduction inmagnetic force due to a change in temperature is suppressed.

The surface of the magnet 1 which is perpendicular to the axialdirection is magnetized, so that the actuator can be thinned as comparedwith an actuator using a cylindrical magnet whose outer peripheralsurface is magnetized. The stator 3 is extended in the axial direction.Therefore, even when a height of turns of wire for the coil 2 isincreased, a distance between the magnet 1 and the stator 3 can beshortened, so that a magnetic resistance is small. The actuator has nota structure in which a set of inner and outer teeth is opposed to amagnet for the transmission of a magnetic flux but a structure in whichthe magnetic flux is allowed to flow from each magnetic pole tooth intothe magnet. As a result, the number of magnetic pole teeth per phase maybe ½ of the number of magnetic poles of the magnetized magnet. Thus, ascompared with a conventional actuator in which the number of magneticpole teeth per phase is equal to the number of magnetic poles of themagnetized magnet, it is advantageous in workability and mechanicalstrength with respect to a width of each of the magnetic pole teeth andan interval therebetween particularly in the case where the number ofmagnetic poles of the actuator is intended to increase.

The magnet 1 is located on an outermost side (outer peripheral side) inthe inner portion of the motor serving as an actuator, so that a radiusof the magnet 1 becomes larger, which is advantageous to convert agenerated force into large torque. In the same view, even when themagnet exists in a location having a small radius, a total percentage ofthe magnet that contributes to increased torque is small. Thus, a widthof the magnet 1 on a surface thereof which is perpendicular to the axialdirection is narrowed and a volume of the coil 2 located inside themagnet 1 is increased according to the narrowed width, with the resultthat space balance efficiency is high and a characteristic of theactuator can be improved. The coil 2 is located at the same height asthat of the magnet 1, so that the actuator can be thinned without anincrease in height in the axial direction.

In addition, the stator 3 has the magnetic pole teeth 3 a to 3 d formedalong the entire circumference at regular pitches. Thus, an area of thestator opposed to the magnet 1 can be increased, with the result that itis possible to maximally use the magnetic fluxes from the magnet 1. Thebearing 4 is made of the soft magnetic material, so that the magneticfluxes passing through the bearing are unlikely to saturate. Thus, thereis no reduction in efficiency.

The coil 2 is surrounded by the stator 3 made of the soft magneticmaterial and the like, so that the amount of leakage magnetic flux fromthe magnetic circuit is small. Therefore, there is no reduction inefficiency. The driving force is caused in a range corresponding to theentire circumference, so that an unnecessary force in a transversedirection is unlikely to cause when the driving force is converted intotorque. Thus, vibration, noise, non-uniform rotation in the actuator areunlikely to cause, thereby obtaining the actuator having high stopposition precision.

According to the first embodiment of the present invention, the rotor 5and the bearing 4 are provided in addition to minimal elements for theelectromagnetic driving apparatus, such as the magnet 1, the stator 3,and the coil 2, so that the number of parts is small. Each element has aflat shape which is simple and easy to produce, so that a manufacturingcost becomes lower.

The axial directional attraction constantly acts between the rotor 5 andthe stator 4, so that the rotor 5 is resistant to disconnect in theopposite direction to the bearing 4. Therefore, it is possible to obtaina structure in which positioning of the rotor 5 in the axial directionand radial direction thereof is performed by using the single bearing 4provided on the stator 3 side and a simple stopper member for the rotor5 is provided in a cover or the like on the opposite side in the axialdirection. When an output portion such as the output pin 5 c is devisedsuch that reaction from a member to be driven is set to a value equal toor smaller than the attraction of the rotor 5, the stopper memberprovided in the cover or the like can be omitted. Thus, it is possibleto further thin the actuator.

Second Embodiment

FIG. 5 is a perspective view showing an actuator according to a secondembodiment of the present invention. Only a point different from thefirst embodiment will be described.

In the first embodiment, each of the stator 3 and the bearing 4 is madeof the soft magnetic material. In the second embodiment of the presentinvention, the stator 3 and the bearing 4 are integrally formed, so thata stator 13 having a bearing portion 13 e is obtained. Such a structurecan be realized by, for example, forging, drawing, or a metal injectionmold manufacturing method.

In FIG. 5, the stator 13 includes not only magnetic pole teeth 13 a, 13b, 13 c, and 13 d but also the bearing portion 13 e and acts to smoothlyrotate the stator 5. Therefore, a process for assembling the bearing andthe stator in the first embodiment is unnecessary, so that a cost can befurther reduced. When the two parts are separated from each other, fourparts, that is, the magnet, the rotor, the bearing, and the stator areinvolved in the precision of a distance between the magnet and each ofthe magnetic pole teeth of the stator, by which a characteristic of theactuator is determined. In contrast to this, in the second embodiment,the bearing (portion) and the stator are integrally formed. As a result,the actuator is composed of the magnet, the rotor, and the stator withthe bearing and the number of parts reduces. An assembly error betweenthe bearing and the stator and a variation in assembly are removed.Thus, a distance between the magnet 1 and each of the magnetic poleteeth 13 a, 13 b, 13 c, and 13 d of the stator 13 is determined withhigher precision, with the result that the distance can be used as ashorter distance at which higher torque can be produced.

When the base plate is not a dedicated plate, a cover 14 may be used.The cover 14 has a cutaway portion 14 a for regulating the rotationalangle of the rotor 5 and is made of a non-magnetic material such asplastic. The cover 14 is located on the same axis as that of the stator13. Therefore, it is possible to prevent the rotor 5 from dropping offand foreign matter from entering the actuator. For example, when afitting hole for a screw is provided in the cover 14, the actuator canbe used as a general purpose product. Other portions are identical tothose in the first embodiment and thus the description is omitted here.

Third Embodiment

FIG. 6 is a sectional view in an axial direction, showing an actuatoraccording to a third embodiment of the present invention, in which astainless steel rod different from that in the first embodiment and thesecond embodiment is used as a shaft of a rotor.

In the case where a large number of magnetic fluxes are not generatedfrom a characteristic of a light quantity adjusting apparatus andmagnetic saturation is unlikely to cause when magnetic fluxes flow froma disk portion 16 a of a rotor 16 to the stator 3, for example, astainless steel rod is used as a shaft 15 of the rotor 16. A softmagnetic material is used for the disk portion 16 a and the like and thestainless steel shaft is produced by press fitting or the like. Otherportions are identical to those in the first embodiment and thus thedescription is omitted here.

Magnetic fluxes around the coil 2 flows through the stator 3 near thecoil 2, the magnet 1, the disk portion 16 b of the rotor 16, the flatportion 4 b of the bearing 4, the bearing 4, and the stator 3 in order.Therefore, the disk portion 16 b of the rotor 16 can be produced at lowcost by performing press-processing or the like on a soft magneticmaterial. A simple and low cost rod material can be used for the shaft15 which serves as the rotational center of the rotor 16 and does notcompose a magnetic circuit, so that a cost of the entire apparatusreduces.

Fourth Embodiment

FIGS. 7 and 8 are a sectional view showing an actuator according to afourth embodiment of the present invention and a top view showing amagnet, a stator, and dummy yokes. As compared with the first embodimentand the second embodiment, a cogging characteristic of the actuator isimproved by incorporating dummy yokes 17 made of a soft magneticmaterial in a portion of a cover 18 which is made of a non-magneticmaterial and used for the prevention of entering of foreign matter andthe fitting of the actuator by a method such as insertion molding.

The cover 18 is molded using high functional plastic generally calledengineering plastic as a non-magnetic material. The dummy yokes 17 madeof electromagnetic soft iron or the like are provided in a part of thecircumference. The positions of the dummy yokes 17 are outside aposition in which the magnetized surface of the magnet 1 is opposed tothe stator 3. The dummy yokes 17 are located at positions which are notin contact with the stator 3 so as not to be influenced by a magneticflux generated by the coil 2 while picking up a leakage magnetic fluxfrom the magnet 1.

According to the insertion into the cover 18, the dummy yokes can belocated at positions in which attraction acts on the magnet 1 and theinfluence of the coil 2 that generates the magnetic flux is small. Whenthe magnet 1 has eight poles, for example, eight dummy yokes areuniformly arranged on the outer diameter side. Therefore, it is possibleto change the cogging characteristic at the time of no energization. Forexample, when the dummy yokes 17 are located in the phase shown in FIG.8, it is possible to increase a retaining force at a standby position ofthe rotor 5. That is, in a case other than the case where the stator 3is excited, torque which acts on the magnet 1, so-called cogging torquecan be adjusted according to a characteristic of the actuator. Morespecifically, retaining torque of the rotor 5 at each of start and endpositions of rotation of the rotor 5 can be increased without a changein a rotational characteristic of the rotor 5 at the time ofenergization to the coil 2. Therefore, in the light quantity adjustingapparatus employing the actuator having the structure according to thefourth embodiment of the present invention (members such as the bladesinvolved in light quantity adjustment are not shown), it is possible toimprove the stability of the blades for blocking the opening portion ofthe base plate at the open position and the closed position. Otherportions are identical to those in the first embodiment and the secondembodiment and thus the description is omitted here.

Fifth Embodiment

FIGS. 9 to 14 show a stepping motor according to a fifth embodiment ofthe present invention. More specifically, FIG. 9 is an explodedperspective view showing the stepping motor. FIG. 10 is a sectional viewin an axial direction, showing the stepping motor. FIG. 11 is aschematic side view showing a phase relationship between a magnet and astator. FIG. 12 is a schematic side view showing a state in which themagnet is rotated by 18 degrees from the state shown in FIG. 11 byswitching of energization to a coil. FIG. 13 is a schematic side viewshowing a state in which the magnet is rotated by 18 degrees from thestate shown in FIG. 12 by switching of energization to the coil. FIG. 14is a schematic side view showing a state in which the magnet is rotatedby 18 degrees from the state shown in FIG. 13 by switching ofenergization to the coil.

In FIGS. 9 to 12, a first magnet 21 having a ring shape has magnetizedportions which are formed by dividing a surface of the first magnet 21which is perpendicular to the central axis thereof into P (P is an evennumber; P=10 in the fifth embodiment of the present invention) in aperipheral direction. The magnetized portions are alternatelyplane-magnetized in the N-pole and the S-pole. Here, the surface of thefirst magnet 21 which is opposed to a first stator 23 is magnetized.Magnetized portions 21 a, 21 c, 21 e, 21 g, and 21 i (21 e, 21 g, and 21i are not shown) are magnetized in the S-pole and magnetized portions 21b, 21 d, 21 f, 21 h, and 21 j (21 f and 21 h are not shown) aremagnetized in the N-pole. The first magnet 21 is formed by compressing abonded neodymium magnet. The first magnet 21 may be formed by injectionmolding or made of a ferrite system material. A second magnet 27 is madeof the same part as that of the first magnet 21. That is, a surface ofthe second magnet 27 which is opposed to a second stator 29 ismagnetized. Magnetized portions 27 a, 27 c, 27 e, 27 g, and 27 i (27 e,27 g, and 27 i are not shown) are magnetized in the N-pole andmagnetized portions 27 b, 27 d, 27 f, 27 h, and 27 j (27 f and 27 h arenot shown) are magnetized in the S-pole.

A first coil 22 is formed by winding a lead wire about a bobbin made ofan insulating material and fixed to the first stator 23 at asubstantially concentric position to the first magnet 21 on an innerperipheral surface side of the first magnet 21. A second coil 28 is madeof the same part as that of the first coil 22.

The first stator 23 is made of a soft magnetic material and produced bya forging press process or the like. The first stator 23 includes:magnetic pole teeth 23 a, 23 b, 23 c, 23 d, and 23 e which are formed ina comb-tooth shape and protrude in a direction perpendicular to thecentral axis thereof; a hole for holding a first bearing 24 describedlater; and a protruding portion for adjusting a phase adjustment cutawayportion 25 a of a case 25 to a phase of the magnetic pole teeth. Thenumber of magnetic pole teeth is ½ of the number of poles (P) of themagnetized first magnet 21, that is, 5. All the magnetic pole teeth 23 ato 23 e of the first stator 23 are excited in the same pole (N-pole orS-pole) by energization to the first coil 22. A second stator 29 is madeof the same part as that of the first stator 23. That is, the secondstator 29 is made of a soft magnetic material and produced by a forgingpress process or the like. The second stator 29 includes: magnetic poleteeth 29 a, 29 b, 29 c, 29 d, and 29 e which are formed in a comb-toothshape and protrude in a direction perpendicular to the central axisthereof; a hole for holding a second bearing 30 described later; and aprotruding portion for adjusting a phase adjustment cutaway portion 25 bof the case 25 to a phase of the magnetic pole teeth. The number ofmagnetic pole teeth is ½ of the number of poles (P) of the magnetizedsecond magnet 27, that is, 5. All the magnetic pole teeth 29 a to 29 eof the second stator 29 are excited in the same pole (N-pole or S-pole)by energization to the second coil 28.

The first bearing 24 is fixed to the first stator 23 to smoothly rotatea rotor 26. The first bearing 24 is made of metal which is a softmagnetic material such as electromagnetic soft iron and produced by asintering manufacturing method or the like. The first bearing 24 has athrust abutment which can be in contact with a thrust abutment of one offlange portions 26 b of the rotor 26. As a result, the first magnet 21and the second magnet 27 are positioned relative to the first stator 23and the second stator 29, respectively, in an axial direction. Thesecond bearing 30 is fixed to the second stator 29 and made of the samepart as that of the first bearing 24. That is, the second bearing 30 ismade of metal which is a soft magnetic material such as electromagneticsoft iron and produced by a sintering manufacturing method or the like.The second bearing 30 has a thrust abutment which can be in contact witha thrust abutment of the other of the flange portions 26 b of the rotor26. As a result, the first magnet 21 and the second magnet 27 arepositioned relative to the first stator 23 and the second stator 29,respectively, in the axial direction.

The case 25 is made of a soft magnetic material and has the cutawayportions 25 a and 25 b for adjusting the first stator 23 to the secondstator 29 to obtain a predetermined phase and step portions for holdingthe first stator 23 and the second stator 29 at predeterminedcoaxiality. Here, a phase shift angle between the cutaway portions 25 aand 25 b is expressed by (360 degrees/2P), that is, 18 degrees.Therefore, the first stator 23 and the second stator 29 are shifted forassembly by 18 degrees in a rotational direction.

A rotor 26 is composed of a shaft portion rotatably supported by thefirst bearing 24 and the second bearing 30, a disk portion 26 a having asurface perpendicular to the shaft portion, and the flange portions 26b. The rotor 26 is made of a soft magnetic material such aselectromagnetic soft iron, having a small magnetic resistance. The firstmagnet 21 and the second magnet 27 are fixed to the rotor 26 such thatdifferent poles are opposed to each other. That is, for example, asshown in FIG. 11, the first magnet 21 and the second magnet 27 arebonded to the rotor 26 such that the magnetized portion 21 a (S-pole)corresponds to the magnetized portion 27 a (N-pole) through the diskportion 26 a of the rotor 26 which is sandwiched therebetween.Boundaries between poles are aligned with each other. When a rotarymagnet in which the two magnets are fixed is actually produced, a diskblank-shaped member made of a magnetic material is bonded to each offront and rear surfaces of the rotor 26 in advance and set formagnetization in a magnetization apparatus. Therefore, magnetic polescan be formed in the magnet. According to such a method, phases of themagnetic poles of each of the two magnets are determined based onarrangement of magnetization yokes of the magnetization apparatus andfixation positional precision of a work. Thus, a process such as phaseadjustment for two poles can be omitted. In this embodiment, the magnetsare magnetized after they are bonded to the rotor 26. A method ofbonding the two magnets magnetized in advance to the rotor 26 may beused.

The stepping motor according to the fifth embodiment of the presentinvention has the two-layer structure in which two actuators, each ofwhich is described in the first embodiment or the second embodiment, arestacked in the axial direction. The inner portion of the rotor to whichthe magnets are bonded is commonly used for two magnetic circuits for anA-phase and a B-phase, so that a combination of the two disk-shapedportions opposed to the stators is generally used as a single disk toreduce the number of parts. A magnetic flux has a characteristic inwhich it flows along, of magnetic paths, a path having a minimalmagnetic resistance, that is, a short formed path. In other words, themagnetic flux passes through a surface of the disk portion to which eachof the magnets is bonded in each phase in a thickness direction of thedisk portion. When the disk portion has a predetermined thickness, themagnetic fluxes flowing through the respective magnetic circuits areunlikely to interfere with each other.

Here, the operation will be described. As shown in the sectional view ofFIG. 10 (hatching indicating the section is omitted), three upper,middle, and lower units are provided. The upper unit is the first stator23 to which the first bearing 24 and the first coil 22 are fixed. Themiddle unit is the rotor 26 whose surfaces (front and rear surfaces) arebonded to the first magnet 21 and the second magnet 27, respectively.The lower unit is the second stator 29 to which the second bearing 30and the second coil 28 are fixed. When the first stator 23 and thesecond stator 29 are closely fixed to the step portions of the case 25,a distance between the thrust abutment of the first bearing 24 and thethrust abutment of the second bearing 24 becomes a predeterminedinterval. This distance is a distance obtained by adding an intervalbetween the upper and lower flange portions 26 b of the rotor 26 to aminimal margin required for rotation. As a result, a gap C between thefirst magnet 21 and the first stator 23 and a gap D between the secondmagnet 27 and the second stator 29 each become a constant interval.Therefore, the air gaps of a gap A and a gap B may be controlled withhigh precision, so that there is no case where the stepping motor ishard to assemble. FIG. 11 shows a state in which the rotor 26 is shiftedto the first bearing 24 side. This indicates a shaft play of a motor(end play) in addition to the predetermined gaps C and D.

As shown in FIG. 11, the magnetic pole tooth 23 a and the like of thefirst stator 23 and the magnetic pole tooth 29 a and the like of thesecond stator 29 are shifted to each other in the rotational directionby an angle of PHs. A phase of a magnetization angle of the first magnet21 is made equal to that of the second magnet 27. Reference L1 denotesan angular pitch between adjacent magnetic pole teeth of each of thestators and M denotes an angle corresponding to a magnetization width ofeach of the magnets. The reference L1 is expressed by (360 degrees×2/P).Here, L is 72 degrees. Reference M is expressed by (360 degrees/P).Here, M is 36 degrees. As a result, PHs is expressed by L1/2, that is,18 degrees.

A group including the first magnet 21, the first coil 22, the firststator 23, the disk portion 26 a, and the first bearing 24 is referredto as an A-phase unit. A group including the second magnet 27, thesecond coil 28, the second stator 29, the disk portion 26 a, and thesecond bearing 30 is referred to as a B-phase unit.

First, a magnetic circuit of the A-phase unit will be described withreference to the sectional view of FIG. 10. When the first coil 22located in the inner circumference is energized, magnetic fluxes aregenerated around the first coil 22. The magnetic fluxes reach thecomb-tooth-shaped magnetic pole teeth 23 a, 23 b, 23 c, 23 d, and 23 eof the first stator 23 which are located near the first coil 22 andopposed to the magnetized surface of the first magnet 21. The magneticfluxes flow from the magnetic pole teeth to the disk portion 26 a of therotor 26 through the first magnet 21. After that, the magnetic fluxesflow from the disk portion 26 a of the rotor 26 to the flange portion 26b which is located in the inner circumference and composes a shaftportion of the rotor 26. Then, the magnetic fluxes flow from the firstbearing 24 to the first stator 23. As a result, the circulation of themagnetic fluxes is completed. The first stator 23, the disk portion 26 aand flange portion 26 b of the rotor 26, and the first bearing 24 eachare made of a soft magnetic material such as SUY (electromagnetic softiron), so that a magnetic loss is small. The first magnet 21 isplane-magnetized, so that the first magnet 21 can be thinned because ithas a sufficient magnetic force even when a thickness thereof does notincrease. The gap C between the first magnet 21 and the first stator 23may be set to a minimal distance required for the rotator and the statorbased on assembly and processing precision. The first magnet 21 is indirect contact with the disk portion 26 a, so that there is no gap.Therefore, the air interval gap C between the first stator 23 and thedisk portion 26 a of the rotor 26 is sufficiently narrow, with theresult that a magnetic loss is small. Thus, a loss of the magneticcircuit which is composed of the first coil 22, the first stator 23, thedisk portion 26 a, and the first bearing 24 is small.

First, a magnetic circuit of the B-phase unit will be described withreference to the sectional view of FIG. 10. When the second coil 28located in the inner circumference is energized, magnetic fluxes aregenerated around the second coil 28. The magnetic fluxes reach thecomb-tooth-shaped magnetic pole teeth 29 a, 29 b, 29 c, 29 d, and 29 eof the second stator 29 which are located near the second coil 28 andopposed to the magnetized surface of the second magnet 27. The magneticfluxes flow from the magnetic pole teeth to the disk portion 26 a of therotor 26 through the second magnet 27. After that, the magnetic fluxesflow from the disk portion 26 a of the rotor 26 to the flange portion 26b which is located in the inner circumference and composes a shaftportion of the rotor 26. Then, the magnetic fluxes flow from the secondbearing 30 to the second stator 29. As a result, the circulation of themagnetic fluxes is completed. The second stator 29, the disk portion 26a and flange portion 26 b of the rotor 26, and the second bearing 30each are made of a soft magnetic material such as SUY, so that amagnetic loss is small. The second magnet 27 is plane-magnetized, sothat the second magnet 27 can be thinned because it has a sufficientmagnetic force even when a thickness thereof does not increase. The gapD between the second magnet 27 and the second stator 29 may be set to aminimal distance required for the rotator and the stator based onassembly and processing precision. The second magnet 27 is in directcontact with the disk portion 26 a, so that there is no gap. Therefore,the air interval gap D between the second stator 29 and the disk portion26 a of the rotor 26 is sufficiently narrow, with the result that amagnetic loss is small. Thus, a loss of the magnetic circuit which iscomposed of the second coil 28, the second stator 29, the disk portion26 a, and the second bearing 30 is small.

Therefore, even when a current flowing into the coil is small, a largenumber of magnetic fluxes can be generated. Thus, a thin stepping motorhaving high output and high efficiency is obtained because strongrepulsion and attraction from the magnet are used.

Next, the rotating operation will be described. As describe above, FIGS.11 to 14 are partial side view showing the phase relationship betweentwelve magnets and two stators in the stepping motor shown in FIGS. 9and 10. For the sake of understanding, the coils and the like are notshown in FIGS. 11 to 14. Hereinafter, the rotating drive of the steppingmotor will be described with reference to FIGS. 9 to 14.

FIG. 11 shows a state in which the first coil 22 is positively energizedto excite the magnetic pole teeth 23 a, 23 b, 23 c, 23 d, and 23 e ofthe first stator 23 in the N-pole and simultaneously the second coil 28is positively energized to excite the magnetic pole teeth 29 a, 29 b, 29c, 29 d, and 29 e of the second stator 29 in the S-pole. In this time,torque is generated to shift the center of each of the magnetizedportions 21 a, 21 c, 21 e, 21 g, and 21 i of the upper surface of thefirst magnet 21, which are magnetized in the S-pole, to the center ofeach of the magnetic pole teeth 23 a, 23 b, 23 c, 23 d, and 23 e of thefirst stator 23 (in a direction indicated by an arrow R in FIG. 11). Inaddition, torque is generated to shift the center of each of themagnetized portions 27 a, 27 c, 27 e, 27 g, and 27 i of the lowersurface of the second magnet 27, which are magnetized in the N-pole, tothe center of each of the magnetic pole teeth 29 a, 29 b, 29 c, 29 d,and 29 e of the second stator 29 (in a direction reverse to thedirection indicated by the arrow R in FIG. 11). Then, the balance in therotational direction is maintained in the state shown in FIG. 11, sothat the rotor 26 stops.

Next, while the first coil 22 is positively being energized, that is,while the magnetic pole teeth 23 a to 23 e of the first stator 23 arebeing excited in the N-pole, when the energization to the second coil 28is switched from the state shown in FIG. 11 to reverse energization, themagnetic pole teeth 29 a to 29 e of the second stator 29 are excited inthe N-pole. Then, torque is generated to shift each of the magnetizedportions 27 b, 27 d, 27 f, 27 h, and 27 j of the lower surface of thesecond magnet 27, which are magnetized in the S-pole, to the center ofeach of the magnetic pole teeth 29 a to 29 e of the second stator 29.Therefore, the rotor 26 starts to rotate in the direction indicated bythe arrow R in FIG. 11. After that, the balance is maintained in thestate shown in FIG. 12, so that the rotor 26 stops. This state is astate in which a combination of the first magnet 21 and the secondmagnet 27, that is, the rotor 26 is rotated in the direction indicatedby the arrow R by 18 degrees from the state shown in FIG. 11.

Next, while the second coil 28 is reversely being energized, that is,while the magnetic pole teeth 29 a to 29 e of the second stator 29 arebeing excited in the N-pole, when the energization to the first coil 22is switched from the state shown in FIG. 12 to reverse energization, themagnetic pole teeth 23 a to 23 e of the first stator 23 are excited inthe S-pole. Then, torque is generated to shift each of the magnetizedportions 21 b, 21 d, 21 f, 21 h, and 21 j of the upper surface of thefirst magnet 21, which are magnetized in the N-pole, to the center ofeach of the magnetic pole teeth 23 a to 23 e of the first stator 23.Therefore, the rotor 26 starts to rotate in the direction indicated bythe arrow R in FIG. 11. After that, the balance is maintained in thestate shown in FIG. 13, so that the rotor 26 stops. This state is astate in which a combination of the first magnet 21 and the secondmagnet 27, that is, the rotor 26 is rotated in the direction indicatedby the arrow R by 18 degrees from the state shown in FIG. 12.

Next, while the first coil 22 is reversely being energized, that is,while the magnetic pole teeth 23 a to 23 e of the first stator 23 arebeing excited in the S-pole, when the energization to the second coil 28is switched from the state shown in FIG. 13 to positive energization,the magnetic pole teeth 29 a to 29 e of the second stator 29 are excitedin the S-pole. Then, torque is generated to shift each of the magnetizedportions 27 b, 27 d, 27 f, 27 h, and 27 j of the lower surface of thesecond magnet 27, which are magnetized in the S-pole, to the center ofeach of the magnetic pole teeth 29 a to 29 e of the second stator 29.Therefore, the rotor 26 starts to rotate in the direction indicated bythe arrow R in FIG. 11. After that, the balance is maintained in thestate shown in FIG. 14, so that the rotor 26 stops. This state is astate in which a combination of the first magnet 21 and the secondmagnet 27, that is, the rotor 26 is rotated in the direction indicatedby the arrow R by 18 degrees from the state shown in FIG. 13.

As described above, when the energization to the first coil 22 and thesecond coil 28 is switched between positive energization and reverseenergization in order, a combination of the first magnet 21 and thesecond magnet 27, that is, the rotor 26 rotates to positionscorresponding to energization phases in order.

According to the structure described in the fifth embodiment of thepresent invention, a stepping motor which is thin in the axial directionwithout a reduction in efficiency is obtained. Magnetic constituentelements are (a) the A-phase stator (magnetic pole teeth), (b) theA-phase magnet and A-phase coil, (c) the rotor disk portion commonlyused for both the A-phase and the B-phase, (d) the B-phase magnet andthe B-phase coil, and (e) the B-phase stator (magnetic pole teeth),which are shown in order along the axial direction. Thus, the steppingmotor can be thinned.

Such a two-phase stepping motor in which two-layer structures, in eachof which the magnet and the coil are fundamentally sandwiched using thestator, are stacked can be produced at low cost and thinned because thenumber of parts is small. In addition, the two-phase stepping motor iseasily assembled because the number of stacks in the axial direction issmall. Those reasons will be described below.

First, the two-layer actuators, each of which is described in the firstembodiment or the second embodiment, are stacked in the axial direction.The two actuators are fixed to the single rotor. The inner portion ofthe single rotor is commonly used for a part of each of the two magneticcircuits for the A-phase and the B-phase.

Secondly, the two magnets, that is, the first magnet 21 and the secondmagnet 27 each having the disk shape are fixed to the disk portion 26 aof the rotor 26 made of the soft magnetic material. The first stator 23excited by the first coil 22 and the second stator 29 excited by thesecond coil 28 are opposed to the magnetized surface of the first magnet21 and the magnetized surface of the second magnet 27, respectively. Asa result, the number of types of air gaps between the stator and themagnets is one for each of the A-phase and the B-phase, that is, two intotal.

Thirdly, the surface of each of the first magnet 21 and the secondmagnet 27 which is perpendicular to the axis as the center of rotationis divided to be alternately magnetized in different poles.

Fourthly, the respective magnetic pole teeth of each of the first stator23 and the second stator 29 are constituted by arranging comb teethextended in a radial direction in the entire circumference.

Fifthly, the first magnet 21 and the second magnet 27 are bonded to therotor 26 such that the phases of the magnetic poles thereof are equal toeach other. The first stator 23 and the second stator 28 are bonded tothe case such that the phases of the magnetic poles thereof are shiftedto each other by an angle obtained by dividing 360° by twice the numberof poles of magnetization.

Sixthly, the first magnet 21 and the second magnet 27 are locatedoutside the apparatus and the first coil 22 and the second coil 28 arelocated inside the inner peripheral surface on substantially the sameaxis at substantially the same height.

Seventhly, the first bearing 24 and the second bearing 30 each are madeof the soft magnetic material.

Eighthly, the number of magnetic pole teeth provided in each of thefirst stator 23 and the second stator 29 is ½ of the number of magneticpoles of the magnetized magnet.

Thus, although the two-phase drive stepping motor has the two-layerstructure in which the two actuators, each of which is the same as thatdescribed in the first embodiment or the second embodiment, are stackedin the axial direction, the rotor is a single part, so that the numberof parts is small. A magnetic flux has a characteristic in which itflows along, of magnetic paths, a path having a minimal magneticresistance, that is, a short formed path. The magnetic flux passesthrough a surface of the disk portion to which each of the magnets isbonded in each phase in the thickness direction of the disk portion, sothat the interference between the magnetic fluxes flowing through therespective magnetic circuits is small.

The magnetic flux generated by the energization to the first coil 22crosses the first magnet 21 located between the first stator 23 and thedisk portion 26 a of the rotor 26, so that an electromagnetic force iseffectively caused. The magnetic flux generated by the energization tothe second coil 28 crosses the second magnet 27 located between thesecond stator 29 and the disk portion 26 a of the rotor 26, so that anelectromagnetic force is effectively produced. The first magnet 21 andthe second magnet 27 are integrally formed with the disk portion 26 a ofthe rotor 26. The first coil 22 is fixed to the first stator 23 and thesecond coil 28 is fixed to the second stator 29. As a result, the numberof types of air gaps between the stator and the magnets is one for eachof the A-phase and the B-phase, that is, two in total. The number ofparts becomes small. The entire stepping motor is composed of the threeportions. Therefore, it is easily assembled and an assembly cost can bereduced. The two magnets are fixed to the front and rear surfaces of thedisk portion 26 a of the rotor 26, so that two blank members each madeof a magnetic material before magnetization can be bonded to the rotor26 and then processed for magnetization by a magnetization apparatus.Thus, it is unnecessary to bond magnetized magnets to the rotor withphase adjustment and assembly becomes easier. For example, a bondedmagnet used as a material of the magnet is generally likely to break.When the bonded magnet is bonded to the rotor 26 made of electromagneticsoft iron or the like, a mechanical strength can be increased. The diskportion 26 a of the rotor 26 also serves as a back metal, so that thestepping motor can be maintained to a magnetically stable state, amagnetic strength thereof is increased, and a reduction in magneticforce due to a change in temperature is suppressed.

The surface of the magnet 21 which is perpendicular to the axialdirection is magnetized, so that the stepping motor can be thinned ascompared with a stepping motor using a cylindrical magnet whose outerperipheral surface is magnetized. Each of the first stator 23 and thesecond stator 29 is composed of comb-teeth extended in the radialdirection, with the result that sizes thereof in the axial direction canbe reduced as compared with that of a stator composed of comb-teethextended in the axial direction. The stepping motor has not a structurein which a set of inner and outer teeth is opposed to a magnet for thetransmission of a magnetic flux but a structure in which the magneticflux is allowed to flow from each magnetic pole tooth into the magnet.As a result, the number of magnetic pole teeth per phase may be ½ of thenumber of magnetic poles of the magnetized magnet. Thus, as comparedwith a conventional stepping motor in which the number of magnetic poleteeth per phase is equal to the number of magnetic poles of themagnetized magnet, it is advantageous in workability and mechanicalstrength with respect to a width of each of the magnetic pole teeth anda gap therebetween particularly in the case where the number of magneticpoles of the stepping motor is intended to increase.

The magnet is located on an outermost side (outer peripheral side) inthe inner portion of the motor, so that a radius of the magnet becomeslarger, which is advantageous to convert a generated force into largetorque. In the same view, even when the magnet exists in a locationhaving a small radius, a total percentage of the magnet that contributesto increased torque is small. Thus, a width of the magnet on a surfacethereof which is perpendicular to the axial direction is narrowed and avolume of the coil located inside the magnet is increased according tothe narrowed width, with the result that space balance efficiency ishigh and a characteristic of the stepping motor can be improved. Thecoil is located at the same height as that of the magnet, so that thestepping motor can be thinned without an increase in height in the axialdirection.

In the two-phase stepping motor, the phases of the two magnetizedmagnets are made equal to each other and the phases of the two statorsare shifted to each other by the angle obtained by dividing 360 degreesby twice the number of poles of magnetization. Therefore, two magnetscan be formed by simultaneously magnetizing two magnetic materialsbonded to the rotor. The two-phase stepping motor using two coils, thefirst coil 22 and the second coil 28 is used, so that control thereof iseasy and an electrical drive circuit is simple. Each of the stators hasthe magnetic pole teeth formed along the entire circumference at regularpitches. Thus, an area of each of the stators opposed to the magnets canbe increased, with the result that it is possible to maximally use themagnetic fluxes from the magnets. Each of the bearings is made of thesoft magnetic material, so that the magnetic fluxes passing through thebearings are unlikely to saturate. Thus, there is no reduction inefficiency.

The coils 22 and 28 are surrounded by the stators each made of the softmagnetic material and the like, so that the amount of leakage magneticflux from the magnetic circuits is small. Therefore, there is noreduction in efficiency. The driving force is caused in a rangecorresponding to the entire circumference, so that an unnecessary forcein a transverse direction is unlikely to cause when the driving force isconverted into torque. Thus, vibration, noise, non-uniform rotation inthe stepping motor are unlikely to cause, thereby obtaining the steppingmotor having high stop position precision.

According to the fifth embodiment of the present invention, the rotorand the bearings are provided in addition to minimal elements for theelectromagnetic driving apparatus, such as the magnets, the stators, andthe coils, so that the number of parts is small. Each element has a flatshape which is simple and easy to produce, so that a manufacturing costbecomes lower.

Sixth Embodiment

FIG. 15 is a sectional view showing a stepping motor according to asixth embodiment of the present invention. As compared with the fifthembodiment, the rotor 26 is divided into two parts, that is, a disk 36and a shaft 37. The disk 36 is made of a soft magnetic material and astainless steel rod or the like is used as the shaft 37. Other portionsare identical to those in the fifth embodiment and thus the descriptionis omitted here.

A magnetic flux generated around the coil flows through the stator nearthe coil, the magnet, the disk 36, the flange portion, the bearing, andthe stator in order. Here, the disk 36 bonded to the magnet and theflange portion each are made of a soft magnetic material such aselectromagnet soft iron. A stainless steel rod material having highrigidity and hardness is used as the shaft 37, so that an available rodmaterial having a high performance and a low cost can be used as theshaft 37. Therefore, when the disk 36 is processed by pressing or thelike, a total part cost reduces. Even in this case, two blank memberseach made of a magnetic material before magnetization can be bonded tothe disk 36 in advance and then processed for magnetization by amagnetization apparatus. Thus, it is unnecessary to perform assemblywith phase adjustment for magnetized magnets.

Seventh Embodiment

FIG. 16 is a sectional view showing a stepping motor according to aseventh embodiment of the present invention. As in the sixth embodiment,an available rod material made of stainless steel is used as the shaft37 of the rotor. However, a rotor 46 to which the two magnets are bondedis divided into two disk portions 46 a. A spacer 48 such as PC isinterposed as an air gap between the two disk portions 46 a. As aresult, the two magnets, the two disk portions, and the spacer areintegrally formed with the shaft 37. Other portions are identical tothose in the sixth embodiment and thus the description is omitted here.

A magnetic flux generated around the coil flow through the stator (23;29) near the coil (22; 28), the magnet (21; 27), the disk portion 46 aof the rotor, the flange portion 46 b, the bearing (24; 30), and thestator (23; 29) in order. Therefore, the A-phase magnetic circuit andthe B-phase magnetic circuit do not interfere with each other in thedisk portions 46 a of the rotor 46, so that output torque of the rotor46 increases. Even in this case, two blank members each made of amagnetic material before magnetization can be bonded to the diskportions 46 a in advance and then magnetized by a magnetizationapparatus. Thus, it is unnecessary to perform assembly with phaseadjustment for magnetized magnets.

Eighth Embodiment

FIG. 17 is a sectional view showing a stepping motor according to aneighth embodiment of the present invention. As compared with the fifthembodiment, bearings 54 and 60 for the rotor 56 each are made of oilmetal of sintered brass which is a non-magnetic material. Based on sucha condition, the shapes of the rotor 26 and the stators 23 and 29 in thefifth embodiment are partially modified to form a rotor 56 and stators53 and 59. Other portions are identical to those in the fifth embodimentand thus the description is omitted here.

A magnetic flux generated around the coil flow through the stator (53;59) near the coil, the magnet (21; 27), a disk portion 56 a of the rotor56, a flange portion 56 b, and the stator in order. Here, each of thebearings 54 and 60 is made of oil metal of sintered brass which is anon-magnetic material. The flange portion 56 b of the rotor 56 isextended to a height of the stator. Therefore, a magnetic circuit inwhich the magnetic flux directly flows from the rotor 56 to the stator(53; 59) is provided. As a result, attraction does not occur between theflange portion 56 b of the rotor 56 magnetized by being bonded to themagnet and the bearings 54 and 60. It is possible to use the bearingmade of oil metal of sintered brass which is more available, so that arotational loss and a cost can be reduced.

As described above, according to any one of the above embodiments of thepresent invention, there can be provided an actuator which maintainshigh efficiency, can react to an increase in pole, has a high part fit,and can be manufactured at low cost while it is thinned, and a lightquantity adjusting apparatus using the actuator as a driving source.

In addition, there can be provided a high performance stepping motorwhich can react to an increase in pole, has a high part fit, and can bemanufactured at low cost while it is thinned.

This application claims priority from Japanese Patent Application Nos.2004-000796 filed Jan. 6, 2004 and 2004-144377 filed May 14, 2004, whichare hereby incorporated by reference herein.

1. An actuator, comprising: a magnet which is formed in a ring shape, inwhich at least one surface perpendicular to a central axis thereof isdivided in a peripheral direction and alternately plane-magnetized indifferent poles; a stator including a plurality of magnetic pole teethopposed to the magnetized surface of the magnet; a rotor which is heldto be rotatable about a rotational axis as a center, includes a diskportion whose surface perpendicular to the rotational axis is bonded tothe magnet, and made of a soft magnetic material; and a coil which islocated on substantially the same surface as that of the magnet, fixedto the stator, and excites the stator and the disk portion.
 2. Anactuator according to claim 1, wherein the magnet is fixed to a surfaceof the disk portion of the rotor which is opposed to the stator on anouter peripheral side and the coil is fixed to a surface of the statorwhich is opposed to the magnet on an inner peripheral side.
 3. Anactuator according to claim 2, further comprising a bearing which isfixed to the stator, rotatably supports a shaft portion of the rotor,and made of a soft magnetic material.
 4. An actuator according to claim3, wherein the rotor is positioned in an axial direction by making a tipprotruding portion of the shaft portion to be in contact with a bottomside of the bearing and the bearing is extended such that a tip endsurface thereof is close to the disk portion of the rotor.
 5. Anactuator according to claim 1, further comprising a dummy yoke which islocated outside the magnet, bonded to a cover made of a non-magneticmaterial, and made of a soft magnetic material.
 6. A light quantityadjusting apparatus, comprising: a magnet which is formed in a ringshape, in which at least one surface perpendicular to a central axisthereof is divided in a peripheral direction and alternatelyplane-magnetized in different poles; a stator including a plurality ofmagnetic pole teeth opposed to the magnetized surface of the magnet; arotor which is held to be rotatable about a rotational axis as a center,includes a disk portion whose surface perpendicular to the rotationalaxis is bonded to the magnet, and made of a soft magnetic material; acoil which is located on substantially the same surface as that of themagnet, fixed to the stator, and excites the stator and the diskportion; an output member which operates in response to rotation of therotor; a base plate including an opening portion; and a light quantityadjusting member which is driven by the output member and moves abovethe opening portion of the base plate to change a quantity of lightpassing through the opening portion.
 7. A stepping motor, comprising: afirst magnet and a second magnet, each of which is formed in a ringshape, in each of which at least one surface perpendicular to a centralaxis thereof is divided in a peripheral direction and alternatelyplane-magnetized in different poles; a first stator including aplurality of magnetic pole teeth which are opposed to the magnetizedsurface of the first magnet and extended in a radial direction; a secondstator including a plurality of magnetic pole teeth which are opposed tothe magnetized surface of the second magnet and extended in the radialdirection; a rotor which is made of a soft magnetic material andincludes a shaft portion held to be rotatable and a disk portion havinga surface perpendicular to the shaft portion; a first coil which isfixed to the first stator and excites the first stator and the diskportion of the rotor to which the first magnet is fixed on one side; anda second coil which is fixed to the second stator and excites the secondstator and the disk portion of the rotor to which the second magnet isfixed on the other side.
 8. A stepping motor according to claim 7,wherein a first flange portion which protrudes from the disk portion tothe first stator is located inside the first coil fixed to the firststator, the first magnet is located outside the first coil, and the diskportion of the rotor is located on an opposite side of the first coilwhich is in contact with the first stator, a second flange portion whichprotrudes from the disk portion to the second stator is located insidethe second coil fixed to the second stator, the second magnet is locatedoutside the second coil, and the disk portion of the rotor is located onan opposite side of the second coil which is in contact with the secondstator.
 9. A stepping motor according to claim 8, further comprising abearing which is fixed to the first stator and the second stator,rotatably supports the rotor, and made of a soft magnetic material, thebearing being connected with the first flange portion and the secondflange portion.